Zinc oxidation process

ABSTRACT

The surface oxidation process of the zinc or of a zinc coating consists of immersion of the zinc in an electrolytic cell, where a direct current of selected magnitude is driven and where the electrolytic bath consists essentially of a solution in an alkali metal nitrate melted mixture either of an hexavalent oxide of the type XO3 or of a salt of the type Y2XO4 or of a salt of the type Y2Cr2O7, where X represents an element selected in the group of chromium molybdenum and tungstenum and Y an element selected between sodium and potassium. When solution of hexavalent chrome CrH3 in a mixture of melted alkali metal nitrates is used as bath, the oxidizing process can be carried out without current in the cell, by mere immersion of the zinc.

United States Patent [191 Gugliemi et al. Dec. 3, 1974 [5 ZINC OXIDATION PROCESS 3,330,744 7/1967 Inventors: Nicolo Gugliemi; Settimo Vittone; Gianfranco Rolando, all of Turin,

Italy Assignee: Ing. C. Olivetti & S.p.A., Torino,

Italy Filed: June 8, 1972 Appl. N0.: 260,796

Chem. LIOB time=20 temp.:30C

No. of hours in salt chamber Example Not Corrosion phase current denty:O,10A/dm Wright 204/56 R Primary Examiner-T. M. Tufariello Attorney, Agent, or Firm-I. J. Schaefer 5 7] ABSTRACT The surface oxidation process of the zinc or of a zinc coating consists of immersion of the zinc in an electrolytic cell, where a direct current of selected magnitude is driven and where the electrolytic bath consists essentially of a solution in an alkali metal nitrate melted mixture either of an hexavalent oxide of the type X0 or of a salt of the type Y XO or of a salt of the type Y Cr O where X represents an element selected in the group of chromium molybdenum and tungstenum and Y an element selected between sodium and potassium. When solution of hexavalent chrome Crl l in a mixture of melted alkali metal nitrates is used as bath, the oxidizing process can be carried out without current in the cell, by mere immersion of the zinc.

C O =8,5% current den sity =4 Aldm time :10

current densily =0 A/dm time :10

[:1 red oxidation white oxidation PATENTED BED 3 I974 No. of hours Chem.L1OB

in time=20' salt temp; 30C

chamber Example N0.

Corrosion phase C 3 1 current densi ty =o,10 A/dm time 10 cr0 =e,5% current density =4 A/dm time :10

cro 10% current density =0 A/dm time :10'

E discolouration phase- ZINC OXIDATION PROCESS BACKGROUND OF THE INVENTION The present invention relates to a method of oxidizing the surface of an article having at least a surface layer of zinc.

One of the most widespread surface treatments for improving the corrosion resistance of steel is that of galvanizing by plating with zinc. It has been found that a longer life for the protective zinc layer may be attained by the anodic oxidation or the passivation of the layer.

Typical conventional methods for anodic oxidation of the zinc involve the use of electrolytes such as a relatively diluted aqueous solution of ammonia, sulforic acid, chromic acid, and ammonium fluoride. The layers attainable by these methods show adequate corrosion resistance, but they lack good mechanical characteristics and require high formation voltages (from 90 to 200 V).

Baths of melted nitrate mixture have been used successfully for the anodic oxidation of aluminum and of the valve-metals (namely, the metals showing a high surface resistivity in the anodic direction and a low surface resistivity in the cathodic direction when used as electrodes in an electrolytic cell, such as aluminum, tantalium, zirconium, niobium). However, such baths cannot be used for zinc inasmuch as the anodic current produces the dissolution of the zinc itself.

Passivation-is normally executed by a chemical process involving immersing the zinc in an aqueous solution having a chromate basis and produces oxide and chromate layers on the zinc surface having a rather complex structure. Such processing imparts a certain atmospheric corrosion resistance to the layer, but the resulting passivated layers are very thin and have little resistance to abrasion.

SUMMARY OF THE INVENTION The disadvantages of the prior methods are obviated by the method of the present invention which provides a new method for the anodic oxidation of a zinc surface in which the electrolytic bath comprises a mixture of melted alkali metal nitrates and/or alkaline earth metal nitrates with the addition of a selected quantity of an oxidizing additive.

According to another aspect of the present invention a zinc surface may be passivated by immersion in a bath comprising a solution of hexavalent chrome oxide CrO in a mixture of melted alkali metal nitrates and alkaline earth metal nitrates.

BRIEF DESCRIPTION OF THE DRAWING The Drawing displays the results of corrosion resistance tests of zinc plated sheet metals processed by the method of the invention.

DETAILED DESCRIPTION In the process according to the invention, the object whose zinc surface is to be oxidized is degreased and immersed in an electrolytic cell wherein the electrolyte comprises a solution of an oxidizing substance in a mixture of melted salts. The salts may be alkali metal nitrates and/or alkaline earth metal nitrates, while the oxidizing substance may be an oxide of the type X or a potassium or sodium salt of the type Na (K )XO where X is an element selected in the group Vla (chromium, molybdenum and tungsten), or a potassium or sodium bichromate salt (e.g., Na (K Cr O The composition of this bath must be such so that its melting temperature is lower than the melting temperature of the zinc.

A current of selected magnitude is passed through the electrolytic cell with the object to be oxidized being used as the anode. The current density may be between 0 to 20 A/dm and the immersion time may be between 30 sec. and 30 min. In these conditions the voltage across the cell does not exceed ten volts.

Grown on the so processed objects is a layer of complex oxides having characteristics depending on the processing parameters. The object is then extracted, cooled and washed with water.

When the percentage by weight of the oxidizing substance is lower than about 2 percent, the processing is insufficient inasmuch as the zinc dissolves without forming surface oxide layers.

When the percentages of the oxidizing substance varies from 2 percent about to about 10 percent and the current density is less than 2 A/dm the oxides attained are iridescent, conductive and very thin, having a thickness less than 1 micron.

Conversely, solid (thickness up to 10 strongly colored, insulating oxides, are attained by baths containing from about 2 to 10 percent of the oxidizing agent and operating with current densities higher than about 2 A/dm The oxides attained by the methodof the invention are made of two well defined layers; an external layer which is variously colored according to the processing undergone and less compact and an internal layer which is light colored, compact and well anchored to the zinc substrate. It is thought that this second layer is probably responsible for the good abrasion resistance found. I

For percentages by weightof oxidizing substances higher than 10 percent the current flow produces a quick dissolution of the zinc.

The consistency and the properties of these two layers are determined by the time of oxidation (whichmay vary from 30 sec. to 30min. as stated above) and the temperature of oxidation (which may vary between about 220C and 280C). By extending the time of oxidation the thickness of the two layers is increased proportionally when the temperature is increased, the external layer takes up varying colorations, while the internal layer substantially increases in uniformity and compactness.

When the sodium chromate Na CrO or the potassium chromate K CrO are used as oxidizing agents, due to their lower solubility, their concentration in the bathcannot exceed about 8 percent.

Furthermore, due to their lower oxidizing power, these oxidizing agents are to be present with concentration not lower than 4 percent.

If the oxidizing agent is constituted by sodium bichromate Na Cr O or potassium bichromate K Cr 0 which have a good solubility, the process can be oper-- ated with a concentration of these salts higher than 10 percent. However, also for them the treatment duration has to be not shorter than 1 min.

A variant of the invention consists in executing the surface processing of the zinc by the passivation process, without the use of electrical current, in a bath made of a mixture of melted salts. The object to be passivated is suitably degreased and immersed in a bath which is substantially the same as the one used as an electrolyte for the anodic oxidation, but with the hexavalent oxide CrO used as oxidizing substance in a percentage equal or higher than the 10 percent. Formed on the so processed zinc is an oxide protection layer which is similar to the ones attained through the anodic oxidation processing by currents greater than 2 A/dm and percentages of additive mixture comprised between the 2 and the 10 percent.

A good surface oxidation processing of the zinc in a melted nitrate mixture resulting in the production of a thick and compact oxide layer can thus be attained in either a pure by chemical or an electrolytic manner by suitably selecting the conditions of the processing, and

in particular, the bath composition. The oxide protection layers attained by the two methods are not perfectly equivalent however. Oxide layers attained by anodic oxidation and layers attained by passivation are both endowed with a good corrosion resistance, but only the first ones have greatly improved mechanical properties such as resistance to abrasion.

For-the tests of the method according to the inven tion which are here described by way of example, baths of sodium and potassium nitrate equi-molecular mixtures melting at a temperature from 218C to 222C were prepared, to which chromic anhydride CrO having a melting point of 196C is added in varying percentages.

A cylindric container of inox steel 18/8 having a capacity of 2.5 l was used as electrolysis cell. The temperature was checked by a chromel-alumel thermocouple immersed in the bath. The anodic oxidation process used as cathods the electrolysis cell itself and as anode thin plates of zinc plated steel, the thickness of the zinc layer being 20p.

The percentage of CrO was varied from to percent, the current density from O to A/dm and the bath temperature from 218C to 250C.

For comparison purposes and for displaying the advantages and some characteristics of the present invention, some tests have been carried out in order to compare the behaviour of thin steel plates coated with a 20 zinc layer which have undergone a traditional processing with the behaviour of similar thin plates processed according to the invention.

The traditional processing consists in suitably degreasing and immersing the thin plate into an 1.2 percent aqueous solution of macro-brite LlOB (chrome passivation and conversion bath of the Ital-Finish, Milan) at the temperature of 30C. After 30 sec. immersion the thin plate is extracted from the bath, washed with cold flowing water and subsequently dried in a flow of warm air.

For the corrosion tests the oxidized thin plates were inserted into a saline fog chamber (NaCl at 5 percent) at the temperature of 35C 3 1C, and their appearance was observed daily. in the course of the test, lasting i000 hours totally, various corrosion statuses were displayed: (I) decoloration; (2) white oxidation (etching of the zinc coating); (3) red oxidation (etching of the steel substrate).

The results of this test are displayed in the drawing, wherein there is shown the duration and the sequence of the varying corrosion stages for four thin steel plates plated with a 20 thick layer of zinc, which are processed and exposed in different ways to the corrosion .snvirsmmsnt- In the drawing it may be observed as the corrosion resistance properties of a thin plate processed in the traditional manner (example 1 does not differ substantially from those of a thin plate which has undergone an anodic oxidation but by a current density lower than 2 A/dm and by a CrO percentage comprised between 2 and 10 percent (example 2). A substantial improve ment of such properties is attained however when the electrolytic current density is greater than 2 A/dm (example 3). As is illustrated in example 4, the corrosion resistance of the zinc oxidation obtained by passivation is further improved, provided that the concentration of ssqua q rflasatsr than 10 E EE L.

Responsible for the excellent .corrosion resistance of the zinc processed by anodic oxidation or by passivation are the protection layers of thick and compact oxidation formed thereon. Layers having such characteristics are thought to be produced by the high oxidizing conditions resulting from the anodic current or to the high CrO concentration.

Conversely, the abrasion tests were executed by the Standard Abrasion Tester, Model 503, of the Taber 1nstruments (North Tonawand, USA) using grinding wheel CS/lO with a load of 500 grs. The abrasion resistance is expressed by the number of revolutions required for conferring an appreciable conductivity on thssla s re ridst q n thss n- The following table displays the results of tests carried out on thin plates processed according to the method of the invention as previously described for increasing values of the additive percentage (CrO in the electrolytic bath. The table illustrates that as the processing according to the invention generally confers increased resistance, whereas the abrasion resistance of zinc oxidized by the conventional method (example 1) is not increased. In particular, it may be observed that the abrasion resistance is improved. the'other conditions being equal, by increasing thebath temperature or the processing duration. Furthermore, example 9 shows that on the contrary the zinc oxidation achieved through passivation is substantially inefficient for improving the abrasion resistance.

Further tests have been carried out by using chromate and bichromate salts as oxidizing agents. Also for these tests a thin steel plate plated with a 20 layer of zinchas been immersed in an electrolytic cell.

Example a) the electrolytic bath is made of a solution at 8 percent of potassium chromate K CrQ, in an equimolecular mixture of sodium nitrate NaNO and potassium nitrate KNO and kept at a temperature of 230C. By driving a current of 1.5 A/dm through the cell for 20 min. a uniform dark green oxide layer is produced on the zinc layer of the steel plate.

Example b) the electrolytic bath is made of a solution at 10 percent of potassium bichromate K Cr O in an equimolecular mixture of sodium nitrate NaNO and potassium nitrate KNO and is maintained at a temperature of 230C. By keeping the zinc plated steel plate in the cell for 10 min. and driving through the cell a current of 2.5 A/dm", on the zinc layer an oxide layer is produced which is dark colored. compact and smooth.

Example 71 of CrO, Current Processing Processing No. of resi- Nov in density temperature duration stance cycle the bath A/dm C on the abrasion tester l 30 20" 2 5,5 4 2 l 8-220 l 400-600 3 5,5 4 248250 [0' 1000-1200 4 8,5 4 2l8220 10' 400-500 5 8,5 4 248-250" 1 100-1400 6 8,5 4 2 l 8220 5 200-300 7 8,5 8 2 l 8-220 5' 500-600 8 8,5 8 2l8-220 l0 300-400 9 l0 228230 10' 50 What is claimed is:

'1. A method of forming an oxide layer on an object having a surface of zinc comprising the steps of:

a. immersing said object in an electrolytic bath including a solution of hexavalent oxide of the type X0 where X is an element selected from the group consisting of chromium molybdenum and tungsten, in a mixture of melted alkali metal salts, said bath having a melting point lower than the melting point of zinc;

b. passing a direct current of constant density through said bath using said object as an anode.

, 2. The method of claim 1, wherein said hexavalent oxide is CrO having a percentage in the solution between about 2 and the 10 percent, the current density is at least about 2 A/dm and the process duration is comprised between 30 sec. and 30 min.

3. The method of claim 2, wherein said mixture of alkali metal salts include sodium nitrate and potassium nitrate in a equi-molecular mixture, said bath having an operation temperature between 220C and 280C.

4. A method of forming a layer of oxide on an object having a zinc surface comprising the steps of:

a. immersing said object in a electrolytic bath comprising a solution of an acid salt of the type Y XO where Y is an element selected from the group consisting of sodium and potassium, and X is an element selected from the group consisting of chromium, molybdenum and tungsten, in a melted alkali metal nitrate mixture, the melting temperature of said bath being lower than the melting point of zinc; and

b. passing a direct current of constant density through said bath using said object as an anode.

nitrate in an equi-molecular mixture, said bath having an operation temperature between 220C and 280C. 7. A method of forming a layer of oxide on an object having a zinc surface comprising the steps of:

a. immersing said object in a bath comprising a solution of a bichromate salt of the type Y Cr O where Y is an alkali metal selected from the group consisting of sodium and potassium, in a mixture of melted alkali metal nitrates, the melting temperature of said bath being lower than the melting temperature of zinc; and

b. passing a direct current of constant density through said bath using said object as an anode.

8. The method, of claim 7, wherein said bichromate salt is K Cr O having in the bath a percentage of at least 2 percent, the current density is at least about 2 A/dm" and the process duration is comprised between 1 min. and 30 min.

9. The method of claim 8 wherein said mixture of alkali metal salts includes sodium nitrate and potassium nitrate in an equi-molecular mixture, said bath having an operation temperature between 220 and 280C.

10. The method of claim 1, wherein the alkali metal nitrates in the electrolytic bath include alkaline earth metal nitrates.

.' UNITED STATES PATENT OFFICE" CERTIFICATE OF CORRECTION Patent-No. 3,852,172 'Da'te'd December 3, 19-74- Inventofls) Nicolo Guglielmi andG ianfranc'o Rolando It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the first page: Patentees should read' -Guglielmi et a1.

after "lnventorsz" it should read --Nico1o Guglielmi, :SettimoVittone and Gianfranco Rolando Turin, both of Italy-'-; in "Foreign'Application Priority Data" delete "June 9-, 1971" and substitute -June 11, 1971--.

Signed and sealed this 21st day of January 1975, y

(SEAL) Attest: I

MCCOY M. GIBSONYJR. c. MARSHALLQDANN Attesting Officer Commissioner of Patents FORM PQ-105O (10-69) USCOMM-DC sows-ps9 I 1L5. GOVE RNMENT HUNTING OFFICE: 1969 O366-334 

1. A METHOD OF FORMING AN OXIDE LAYER ON AN OBJECT HAVING A SURFACE OF ZINC COMPRISING THE STEPS OF: A. IMMERSING SAID OBJECT IN AN ELECTROLYTIC BATH INCLUDING A SOLUTION OF HEXAVALENT OXIDE OF THE TYPE XO3, WHERE X IS AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF CHROMIUM MOLYBEDENUM AND TUNGSTEN, IN A MIXTURE OF MELTED ALKALI METAL SALTS, SAID BATH HAVING A MELTING POINT LOWER THAN THE MELTING POINT OF ZINC; B. PASSING A DIRECT CURRENT OF CONSTANT DENSITY THROUGH SAID BATH USING SAID OBJECT AS AN ANODE.
 2. The method of claim 1, wherein said hexavalent oxide is CrO3 having a percentage in the solution between about 2 and the 10 percent, the current density is at least about 2 A/dm2 and the process duration is comprised between 30 sec. and 30 min.
 3. The method of claim 2, wherein said mixture of alkali metal salts include sodium nitrate and potassium nitrate in a equi-molecular mixture, said bath having an operation temperature between 220*C and 280*C.
 4. A method of forming a layer of oxide on an object having a zinc surface comprising the steps of: a. immersing said object in a electrolytic bath comprising a solution of an acid salt of the type Y2XO4, where Y is an element selected from the group consisting of sodium and potassium, and X is an element selected from the group consisting of chromium, molybdenum and tungsten, in a melted alkali metal nitrate mixture, the melting temperature of said bath being lower than the melting point of zinc; and b. passing a direct current of constant density through said bath using said object as an anode.
 5. The method of claim 4, wherein said salt is K2CrO4 having a percentage in the solution between about 4 and 8 percent, the current density is at least about 2 A/dm2 and the process duration is comprised between 1 min. and 30 min.
 6. The method of claim 5, wherein said mixture of alkali metal salts includes sodium nitrate and potassium nitrate in an equi-molecular mixture, said bath having an operation temperature between 220*C and 280*C.
 7. A method of forming a layer of oxide on an object having a zinc surface comprising the steps of: a. immersing said object in a bath comprising a solution of a bichromate salt of the type Y2Cr2O7, where Y is an alkali metal selected from the group consisting of sodium and potassium, in a mixture of melted alkali metal nitrates, the melting temperature of said bath being lower than the melting temperature of zinc; and b. passing a direct current of constant density through said bath using said object as an anode.
 8. The method of claim 7, wherein said bichromate salt is K2Cr2O7 having in the bath a percentage of at least 2 percent, the current density is at least about 2 A/dm2 and the process duration is comprised between 1 min. and 30 min.
 9. The method of claim 8 wherein said mixture of alkali metal salts includes sodium nitrate and potassium nitrate in an equi-molecular mixture, said bath having an operation temperature between 220* and 280*C.
 10. The method of claim 1, wherein the alkali metal nitrates in the electrolytic bath include alkaline earth metal nitrates. 